A typical handhold radio telephone typically operates in a frequency range of about 900 MHz in a GSM 900 cellular system. However, a radio telephone may also operate in a satellite radio communication system as well as in other cellular systems like GSM 1800, D-AMPS 800, D-AMPS 1900, DCS 1800, PCS 1900, PDC 800 and PDC 1500. Besides receiving electromagnetic energy in a particular frequency range, the device, via its antenna, also radiates within the corresponding frequency range.
A therefor handheld telephone, which herein after is referred to as a Mobile Phone, includes all portable equipment which can be used for radio communication such as mobile phones, communicators, so called organizers, or the like.
The radiation from such a Mobile Phone must primarily illuminate the horizontal half-plane comprising directions outward from the head and body of an operator. Since a Mobile Phone during a call is very close to the user, the body of the user, especially the hand and the head of the user, will influence the signalling properties of the antenna of the Mobile Phone. Consequently, the illumination of the user is desired to be as low as possible in order to direct the output power from the Mobile Phone towards the most suitable base station with a directional antenna having a cardioid shaped pattern to get the best possible communication and also the best utilization of the battery energy.
The directional antenna must be in harmony with the small size of the handheld modern Mobile Phones. A reasonable size of a 900 MHz antenna would be, for instance, height.apprxeq.40 mm and diameter .apprxeq.12 mm or less. The Mobile Phone must be ready to be used without any extra measures to be taken.
How could we describe the electromagnetic field interacting with the operator? The distance between the Mobile Phone, including it s antenna, and the head of the user often is less than a wavelength. This means that the operator is in the reactive field region or in the radiating near-field region of the Mobile Phone and its antenna. This interaction between the user and the near-field region of the antenna will in general cause deteriorated antenna properties, which mainly depends on absorption and reflection losses. The losses will increase as the distance between the user and the antenna decreases.
In the reactive near-field region the reactive components of the electromagnetic fields are very large with respect to the radiating fields. The reactive fields do not radiate, but are an essential part of the radiating mechanism. The reactive field components decay in a direction from the source with the square or cube of the distance and are generally negligible relative to the radiating fields at a distance greater than a wavelength from the source.
The reactive fields in the vicinity of an antenna can be compared with the reactive field in a transformer and is described by Maxwell's second equation: ##EQU1##
A transformer without an external load and without any internal losses will only have a reactive field, but if a resistive load is applied on the secondary side, an active power will be transmitted to the load, which means that an active field parallel to the reactive fields will be introduced. An analogous situation is found when an operator puts the head close to a Mobile Phone, which is surrounded by reactive near-fields. This means that the head will act as a load to which active power will be transmitted and where the reactive near fields are an essential part of the radiating mechanism.
How could the illumination to the operator be reduced? At least so much that a cardioid shaped pattern resulting in some extra dB antenna gain in the direction outward from the head of the operator could be obtained.
This depends on two conditions:
a) The construction and size of the Mobile Phone. PA1 b) The size and type of the antenna proposed.
If any part of the Mobile Phone, the chassis, the plastic cover if metallized, any of the printed circuit boards or anything else in the device having a resonance frequency inside a frequency band in which the Mobile Phone is supposed to operate, it will cause a problem to control the reactive fields. Then the theoretical mathematical expression for the radiating near-field or the field far away will be very complicated to solve, specially when it is combined with a proposal to use a miniaturized antenna, for example a miniaturized monopole antenna close to the printed circuit boards or chassis of the phone without any isolating ground plane in between. This results in induced currents, first of all in the resonant parts of the phone, as well as creating an extra interaction with the user and reduced antenna efficiency and thereby also an extra power loss.
The most simple solution of the problem can be used if the Mobile Phone structure has no resonances in any of the frequency bands in which the device is to be used, and a full length end-fed half wave antenna is proposed. What cannot be achieved in this case, without making the antenna construction still more bulky, is the directivity and gain in the zone outward from the head of the operator.
This type of problems with their different basic conditions can not be solved with one single idea or one single principle, but by means of a group of ideas and principles, where sometimes the whole group of ideas must be combined and sometimes only a single principle may be used.
The most general solution is to apply a passive reflector behind the antenna radiator. Such a construction, for instance, is disclosed in the U.S. Pat. No. 5,335,336, "Radiation shielding apparatus for a radio transmitting device", by J. Daniels, 1994. Additionally there are available several different approaches to this problem for instance U.S. Pat. No. 5,338,896 and U.S. Pat. No. 5,367,309, both from 1994. Still another solution is presented in the International Publication WO 95/31048, just to mention a few. The reflector must have a correct length and a magnitude of the spacing between the radiator and the reflector of preferably 0.1 to 0.15.times..lambda..
As radiator a monopole and some sort of ground plane and a counterweight will be needed. If the desired radiator is a miniaturized end-fed half-wave antenna, which for instance means a helix shaped radiator with a total mechanical length somewhat shorter than a quarter of a wavelength, this will imply that it is also necessary to reduce the length of the reflector to keep the total length of the antenna system within a reasonable measure, e.g., 0.2.times..lambda. or less. It is also desired that a ground screen, or part of it, is integrated in the antenna unit. This integration may be realized by connecting a parallel line choke or a small conical, and coil to the screen of the coaxial antenna connector as is demonstrated, for example, in the Swedish Patent Application 93 02420-6.
To conclude it is evident that there is an increasing demand for further development in the area of increasing the antenna efficiency of a handhold Mobile Phone, particularly for designs which uses miniaturized antennas, to minimize the interaction with the user and at the same time presenting a device which is also always ready for use without any extra manipulation to obtain the necessary antenna function.